The present invention relates to gas turbine engines (gas or steam) and more particularly to heat exchanger arrangements within such turbine engines utilised to provide ventilation air or for cooling of other fluids such as fuel or oil or compressed air utilised for de-icing.
FIG. 1a is a schematic side view of an engine showing a prior art heat exchanger arrangement and FIG. 1b is a plan view in the direction of arrowhead A of the engine and in particular the heat exchanger arrangement as depicted in FIG. 1a; Compressed air bled from the compressor stages of a gas turbine engine is utilised to provide ventilation air within the cabin of an aircraft associated with a gas turbine engine. This air must be cooled for acceptability with respect to such ventilation or other functions. Typically, the heat exchanger acts as a pre-cooler taking fan by-pass air flow to act as a fluid flow for coolant through the heat exchanger. This fluid flow after acting as a coolant within the heat exchanger in some turbine engine configurations is exhausted into an engine compartment or zone. Previously, as illustrated in prior art FIGS. 1a and 1b, the heat exchanger arrangement 1 comprises a heat exchanger 6 which is presented with a fluid flow 2 from a Fan compressor stage 3 and a guide vane 4. The heat exchanger 1 is housed within a bifurcation 9. The fluid flow 2 passes through an air fan valve 5 to regulate the flow rate into the heat exchanger 6 where as indicated a cooling action occurs with respect to a bled compressor air flow taken through ducting 7 shown in broken line. The exhaust fluid flow 8 exits into a vent zone 11 where it is mixed with gas flows, including ventilation air flow 101, through the engine. Such an approach has a number of problems including increase in the temperature in the zone 11 and as a result of the large exit area results in a drag effect within the zone 11 detrimental to operation.
In view of the above, although exhausting fluid flow 8 into the zone 11 is mechanically simple it creates a number of problems. The zone vent exit 12 has to be sized to deal with the combined usual zone ventilation flow 101 through a central cowling 13 of the engine as well as the highest potential exhaust flow 8, and flight conditions when the heat exchanger 6 is not operating. Furthermore, thrust recovery is compromised from the vent zone 11 in view of the size of the vent 12 which is effectively over-sized through normal operation for the reasons described above. It will also be understood that this over-sized vent 12 creates a drag penalty because the vent acts as an aerodynamic step or discontinuity when the exhaust flow 8 is not flowing. A further disadvantage as indicated above is that there is extra heat input to the zone 11 which may require considerable shielding and heat resistant cabling within the core 13. It will also be understood the variability with regard to the exhaust flow 8 makes tuning of the flow regimes in the event of a fire extremely difficult in order to maintain that the extinguishants achieve a required density in all parts of the zone 11.